1. Field
Apparatuses consistent with the exemplary embodiments relate to hard disk drive (HDD) technology, and more specifically, to an HDD actuator flexible circuit.
2. Related Art
FIG. 1 illustrates an HDD with a dynamic loop 110. HDDs in the related art may utilize a flexible circuit connecting the heads 107 on an actuator arm 104 to the printed circuit board (PCB) 103. The flexible circuit moves freely and dynamically as the actuator arm moves between inner and outer data cylinders on the disk platter. The flexible circuit serves to facilitate instructions and functions between the PCB and the actuator arm voice coil motor (VCM) 105, preamplifier 106, and heads 107. The dynamic loop 110 is the portion of the HDD actuator flexible circuit routed from the PCB fixed end 102, also called a connector, to the moving end 101 and the actuator arm 104. Traces are routed onto a flexible material so that when the actuator arm moves while performing functions with the hard disk drive platter (e.g. read, write, seek), the dynamic loop 110 may flexibly move along with the actuator arm while maintaining a connection between the PCB 103 and the actuator arm 104 and heads 107. The dynamic loop may be arranged as a single layer circuit or a dual layer circuit.
FIGS. 2A and 2B illustrate cross sections of a single layer dynamic loop and a dual layer dynamic loop, respectively.
In the related art implementations of the single layer dynamic loop, the traces are routed on one side of a flexible material. The single layer circuit may contain a cover polyimide, a cover adhesive, a trace layer which may contain copper traces, and a base polyimide. However, as HDDs become progressively smaller, width and spacing limitations may cause the single layer dynamic loop to consume proportionally more space within the HDD than is desired. To ensure sufficient electrical connectivity between the actuator arm and the PCB, the single layer circuit may have trace width and spacing requirements depending on the design of the HDD. If extra traces are needed between the PCB and the actuator arm to accommodate added functionality, for example dual heaters, dual writers, and laser heat assist writing, the single layer dynamic loop may require more space within the HDD.
A dual layer dynamic loop could be employed to increase the number of traces routed through the dynamic zone while saving space within the HDD. In a dual layer circuit implementation, two layers of traces are routed on the top and the bottom of the flexible material of the dynamic loop. A dual layer dynamic loop may contain a base polyimide, a layer of traces on each side of the polyimide which may contain copper traces, a cover adhesive on each trace layer and a cover polyimide on each cover adhesive layer.
However, the dual layer dynamic loop suffers from additional copper trace stress in comparison to the single layer dynamic loop. As the actuator arm moves while performing functions of the hard disk drive, the dual layer dynamic loop undergoes stress on the top traces and the bottom traces, which may be about twice that of a single layer dynamic loop.
FIGS. 3A and 3B illustrate a stress comparison chart between a single layer dynamic loop and dual layer dynamic loop, for the example configurations in FIGS. 2A and 2B. As shown in FIGS. 3A and 3B, the dual layer dynamic loop configurations undergo roughly twice the amount of peak stresses as the single layer dynamic loop. From FIG. 2A (single layer), six traces have designated names for stress on the top and the bottom, for example, 3t is the maximum stress at the top side of trace number 3. FIG. 2B shows a similar naming convention for a dual layer configuration, and three added traces on the lower Cu level are shown for illustration purposes. FIG. 3A then illustrates the stresses at each position, and for both single and dual layer. FIG. 3B shows the averaged, maximum stresses for overall, top, and bottom trace locations. Due to the greater number of traces running through the dual layer dynamic loop and more so, their geometric distance from the neutral axis, the resulting stiffness of the dual layer dynamic loop configuration is on the order of five times that of the single layer dynamic loop. The table at upper right of FIG. 3B shows simulated values of this. Though the two trace layers of the dual layer dynamic loop can be routed as needed within the HDD, the aforementioned 2× the stress and 5× the stiffness are prohibitive.
Increased functionality of the HDD actuator flexible circuit requires a greater number of traces within the dynamic loop. The increased number of traces may widen the loop and increase stiffness. Therefore, there is a need to route a larger number of traces without sacrificing trace width and spacing while keeping stiffness low and addressing the above-noted stress penalty of dual layer dynamic loops.